Wednesday, August 24, 2011

Diel Vertical Migration (DVM)

Every evening the ocean experiences a dramatic change. As the sun begins to set, many species of zooplankton and fish come to the surface waters to feed. It is said to be the largest migration on earth in biomass and number of animals participating. It is called diel vertical migration (DVM). The word diel comes from the Latin for "day." There is an upwards migration in the evening and a downwards migration in the morning.

There are a few hypotheses as to why this is done, such as predator avoidance. It may be a trade off between the safety of the dark deep sea and the bounty of food, such as phytoplankton, at the surface. Phytoplankton occurs only in water shallow enough to for the sun's rays to reach. Another hypothesis is that it is more bioenergetically efficient to perform DVM than to just stay consistently at the surface, because metabolic rates will be lower in the cooler deep waters.

DVM is also important to understand from a chemical stance. The critters migrate up to the surface waters and proceed to eat all night. Because they return to depth during day, they bring carbon and nutrients down with them, digesting the food, and excreting those nutrients into the deep waters. This migration occurs very quickly; planktonic organisms have been known to travel up and down in the water column at more than 200m per hour (Wiebe 1990). Our preliminary results from this cruise are similar. Swimming 200m/hour for a 1cm planktonic organism is equivalent to a 6ft human swimming 36km per hour, six times faster the Michael Phelps' world record time!

Cuvierina columnela, an example of a pteropod we have been seeing that is known to vertically migrate. Photo: N. Copley.

We are interested in studying the DVM of pteropods! One of the key questions we're trying to answer in this project is whether the DVM of pteropods is affected by ocean acidification. By measuring their DVM in different regions with different chemical conditions, we can determine whether the depth to which the pteropods migrate is shallower in regions where pH is lower in the deep waters where they would usually migrate to. This is why some of our scientific stations are 18 hours long. At these “day-night” stations, we can perform each type of deployment twice; once at night, and once during the day – and compare the results. We perform day and night MOCNESS tows, HammarHead deployments, VPR casts, and deep CTD casts (see post from August 12 for a more detailed explanation about this equipment).

As the MOCNESS tow can sample the water column at a series of discrete depth intervals, we are able to detect these vertical shifts by looking at the species composition of the samples. For example, if we were to find many organisms of the same species in the 200 – 400m sample during the daytime tow, and at night the same species was found more frequently in the shallower tows, it might be indicative that that species performs a diel vertical migration.

We can also study DVM with acoustics. Plotting the acoustic backscatter data for one day allows us to see both the daytime, and the nighttime migration. In the figure below, you can see this inside the black ovals. You can also see a narrow constant band at around 200m; not all species perform these migrations.

An echogram illustrating DVM. See previous post for tips on reading an echogram!

This phenomena is not unique to the north Atlantic. It has been observed in all regions of the ocean and even lakes!